The short answer is intensity (measured in Gauss), when researching PEMF it's easy to get caught up in frequencies and waveforms but most devices available have a similar range of frequencies and waveforms, the way the devices differ the most is the intensity (Gauss).
There is a noticeable difference between the effects of using low or high intensity, both are effective however higher intensity therapy (50 Gauss+) is much stronger and will generally yield faster results.